Parallel radiation such as sunlight may be concentrated with lenses or contoured surface reflectors, as is well understood. A trough reflector with a parabolic cross section concentrates sunlight on a line and a parabolic dish reflector concentrates sunlight on a point. Both types of solar reflectors are well understood in the art. Since sunlight radiation is essentially a remote source of parallel light waves, the reflected waves become concentrated at the focal point, or line, of the reflector. The resulting concentration of energy is impressive. A relatively small dish reflector or convex lens positioned under a bright sun can easily generate combustion temperatures at its focus point.
The book, “A Golden Thread: 2500 Years of Solar Architecture and Technology” by Ken Butti and John Perlin: Cheshire Books, 1980, provides an early industrial example of this technology. In 1912, an array of very large parabolic trough collectors, 13 feet in width, were built in Meadi, Egypt, a small farming community on the Nile River 15 miles south of Cairo. Each collector was 204 feet in length. There were five in all. Each collector was fitted with a mechanical tracker which kept it automatically tilted properly toward the sun.
The heat generated in water pipes running along the line of focus of this reflector array was used to produce steam which drove a series of large water pumps. Together they produced the equivalent of 55 horsepower. They were capable of pumping 6000 gallons of water per minute, bringing irrigation water to vast areas of arid desert land. World War I interrupted this early industrial experiment, but the burgeoning present day solar industry has long since eclipsed this early milestone with sophisticated systems and large installations delivering megawatts of solar-derived electric power, as well as millions of rooftop solar hot water systems supplementing household heating requirements around the world. Today, much of the present solar power industry is directed towards photovoltaic technologies, but parabolic dish and trough type solar reflectors remain an important primary component in many installations for directing a greatly intensified sunlight beam on a relatively small target collector.
The quality and efficiency of the reflective surface of a reflector is obviously important. 95.5% is about the highest level of reflection practical for extended outdoor uses, which is 10-20% more reflective than normal glass mirrors, aluminum foil and Mylar reflector film. Total spectral reflectivity of commercially available reflector panels for the light spectrum between 450 and 2500 mn is at or above 93.5% at a 55-70 degree light angle.
Inexpensive reflective sheet material can be cut and attached to flat, rigid and smooth substrates of wood or metal to form a single pane or faceted reflector, by relatively unskilled persons using no more than basic hand tools. Such reflectors can be positioned by trial and error to obtain a notable redirection and/or concentration of solar energy whether for higher light intensity or heating purposes.
“Lighting sheet” is a general industry term used to describe high quality reflective aluminum produced to meet specific optical properties, used in various lighting applications. Its reflective quality may be inherent or the result of a surface treatment or coating or an additional layer. The base material needs to be further specified to meet the associated structural requirements. Sheet stock that is structurally suitable but not highly reflective may be adapted by application of a thin Mylar film which may be obtained in roll form and applied with an adhesive to the sheet stock before or after it is formed, preferably before.
Multiple simple, flat reflectors can be directed to reflect light to a common collection point. A flat multi-mirror concentrating reflector array called a heliostat can duplicate a section of a parabolic reflector and focus flat sunlight reflections to a single collection point, albeit with less concentration than a curved reflector can achieve. Of course, more and smaller flat mirrors enable a more concentrated and smaller collection point than fewer larger mirrors.
Convex or concave support structures can be calculated and fabricated, to which flat or flexible reflective sheet material can be cut and fitted to form convex or concave semicircular reflectors such as parabolic or constant radius trough reflectors that focus collected light on a collector positioned at the focal point of the reflector. A reflector support can easily be figured with a one or two axis, manual or automatic tracker mechanism to keep the reflector pointed directly at the sun. The collector in turn, such as a pipe or conduit containing a thermal transfer fluid, or a photo electric device or array, can then be connected to an energy conversion device such, as a steam engine, suitable for applying the available power. Fabrication of efficient semicircular trough type reflector systems with a degree of precision normally requires a somewhat more sophisticated set of instructions, materials, tools, time, and/or a more skilled fabricator. In a commercial setting or context, this equates to additional cost.